Acetic Acid Oxidation by Escherichia Coli and Aerobacter Aerogenes

Abstract

Evidence against the occurrence of the Krebs oxidation cycle in bacterial respiration has been steadily accumulating. Escherichia coli and many other bacteria do not readily metabolize the three tricarboxylic acids. Aerobacter aerogenes will not readily attack citrate when measured manometrically unless the organism is grown in the presence of this acid as the sole source of carbon. Recently Lenti (1946) was able to show inhibition of succinic acid oxidation in E. coli by malonate, but the oxidation of pyruvate was not affected. Karlsson and Barker (1948) obtained evidence against the tricarboxylic acid cycle in Azotobacter agilis. There is, therefore, little support for the assumption that the cycle occurs in those bacteria whose intermediary metabolism has been studied in detail. On the other hand, many bacteria, including E. coli and A. aerogenes, oxidize succinate, fumarate, and malate, and reduce anaerobically oxalacetate to succinate, and this suggests that the Szent-Gy6rgyi system, which constitutes an integral part of the Krebs cycle, is operative in microorganisms. By the use of arsenious oxide and cyclohexanol it has been possible to show that glucose or pyruvate is oxidized aerobically as far as acetic acid without the mediation of the C4 dicarboxylic acids as catalytic hydrogen carriers. In other words, the possibility of pyruvic acid initially condensing with oxalacetate to form procitric acid or some other C7 intermediate in its oxidation scheme is ruled out. By applying the principle of "simultaneous adaptation" (Stanier, 1947), it was observed that acetate that arises from the breakdown of either glucose or pyruvic acid is further oxidized to CO2 and water without the mediation of cis-aconitate or a-ketoglutarate. These results may in turn be used as evidence against the occurrence of the tricarboxylic acid cycle in the organisms under consideration. METHODS E. coli or A. aerogenes was grown for 16 to 18 hours, with constant aeration, at 30 C in a medium containing 0.8 per cent substrate, 0.4 per cent (NH4)2SO4, 0.8 per cent KH2PO4, 0.2 per cent yeast extract or 0.07 per cent tryptone, and 20 per cent tap water, at an initial pH of 6.8 to 7.0. The cells were then harvested, washed several times with cold distilled water, and stored in the icebox, or first lyophilized and stored in the dry state. The results vary somewhat, particularly with respect to acetate oxidation by A. aerogenes, depending upon the treatment of the cells. The Barcroft-Warburg respirometer technique was used. The total volume per flask varied from 2.3 to 2.8 ml. Endogenous values were subtracted from those

Studies on the Mechanism of Acetate Oxidation by Bacteria

“…Further, acetate-grown Aerobavter showed a heightened activity on succinate, fumarate, malate, oxalacetate, and pyruvate, but not on a-ketoglutarate, cis-aconitate, or citrate. Similar results were obtained with E. coli (Ajl, 1950;Ajl and Kamen, 1951). The suggestion was put forth (Ajl and Wong, 1951), therefore, that A. aerogenes, when grown on acetic acid, may oxidize the C~-fatty acid in a manner similar to that of E. coll.…”

“…Based upon these findings and those previously published (Ajl and Wong, 1951;Ajl, 1950;Ajl and Kamen, 1951), the following may be considered to be the terminal oxidation pattern of A. aerogenes. Glucose-and glycerol-grown cells will oxidize these and other products to the stage of acetate and stop there.…”

“…Glucose-and glycerol-grown cells will oxidize these and other products to the stage of acetate and stop there. It has been shown (Ajl, 1950) that glucose-grown cells do not oxidize acetate and that acetate accumulates during glucose oxidation. When the same organism is grown on acetate, it will oxidize the C~-acid chiefly by a mechanism best reconciled with a Knoop-Thunberg condensation reaction as shown by the isotope and adaptation data.…”

“…In paper IV of this series (Ajl and Wong, 1951), it was shown that citrategrown Aerobacter aerogenes may utilize simultaneously a tri-and dicarboxylic acid cycle for acetic acid oxidation. From adaptation data, however, it appeared that acetate-adapted Aerobacter behaves more like Escherichia coli; i.e., breaks down acetate d a a pathway best reconciled with a Knoop-Thunberg condensation mechanism (Ajl, 1950;Ajl and Kamen, 1951). The adaptation experiments revealed that whereas citrate-grown cells adapted themselves to oxidize acetate rapidly, the reverse was not altogether true; i.e., acetate-grown Aerobacter was not adapted to oxidize citrate rapidly.…”

Aspartic Acid Deamination by Intact Proteus X‐19

“…This compound was tested originally as a specific inhibitor of fumarase (Woolf, 1929;Ajl, 1950). However, fumarase in whole cells or in extracts was inhibited only at concentrations of cyclohexanol, which affected also many other enzymatic reactions.…”